Not applicable.
The present invention relates to metering valves and in particular to high precision fluid metering valves.
Solenoid type metering valves are well known. Such valves include an inductive coil that when energized attracts or repels an armature which in turn moves a valve member to control flow, for example, of hydraulic fluid or fuel. Solenoid type metering valves are beneficial in that they are relatively simple to design and include a limited number of movable components. Proportional solenoid type metering valves are also well known. Proportional valves control flow rate in proportion to the input signal (current) supplied to the coil which is desirable for accurate control of the valve. Conventional proportional valves can perform quite well for many applications.
However, in precision metering applications, such as when metering fuel and other combustible media to jet and rocket powered vehicles or in applications where the valve is used as an actuator positioning device, the valves must reliably provide consistent and responsive pressure and flow control. In particular, they must be accurate over a wide range of flow rates (high turn-down ratio) and have minimal internal leakage, low power consumption and low hysteresis. They must also be compact.
Solenoid type metering valves operate by generating magnetic flux which pulls an armature to move the valve. The attractive force of magnetic flux on a metallic body becomes stronger and more non-linear the closer the body is to the source of the flux. Large gaps between the armature and the coil require high current levels and/or prohibitive large coils. Thus, in compact valves where the armature is in close proximity to the coil, the position of the armature, and thereby the valve, will vary non-linearly or non-proportionately with the input signal to the coil. This non-linearity tends to make the valves bi-stable as the air gaps between the armature and the coil decrease. This can lead to large fluid pressure oscillations and undesirable instability making the valve inaccurate and difficult to control with precision. Accordingly, the armature movement must be linearized in some manner. However, this can be complicated because a simple linear spring acting on the armature will not maintain proportional movement throughout its usable range.
There have been many means of linearizing the force acting on the armature in compact packages. One known means is to use conically shaped openings for the air gaps between the armature and the coil. However, this can impart relatively large side loads on the armature leading to high friction and poor hysteresis. Low-friction guides or suspension systems for the armature can be used to reduce the side loads, such as in U.S. Pat. Nos. 3,861,643 and 4,635,683, however, they add cost and can be difficult to implement. The ""643 patent discloses another means of providing a valve by the saturation of the magnetic flux at different sized air gaps in the core of the armature. However, as mentioned, this technique requires a complex frictionless suspension system.
Accordingly, an improved precision proportional solenoid type metering valve is needed.
The present invention provides an improved proportional solenoid-operated device for accurately metering fluid using a unique piece-wise non-linear spring to rectify induced movement from magnetic flux that is not proportional to the input signal. In particular, the invention includes a housing containing an electromagnet coil for translating an armature to move a metering valve. The movement of the armature, and thereby the metering valve, is linearized to the coil input signal by the non-linear spring applying counteracting forces on the armature in a stepped or piece-wise manner.
Specifically, the housing defines a valve chamber in fluid communication with an inlet port and an outlet port. The electromagnetic coil in the housing produces magnetic flux varying non-proportionally to an input signal. The armature can be translated by the induced magnetic flux along the stroke axis toward the coil. The metering valve can be moved along the stroke axis by the armature. The non-linear spring communicates with the armature and has a plurality of spring fingers extending radially with respect to the stroke axis to independently engage between the armature and a stationary structure at different points as the armature is translated along the stroke axis. This provides a summing of forces from each spring finger acting on the armature counter to the force induced by the magnetic flux so that positioning the metering valve is more nearly a linear function of the input signal to the coil.
At least one of the spring fingers has a different thickness so that thicker spring fingers deflect before thinner spring fingers.
In a preferred form, the non-linear spring is an assembly of flexure springs spaced apart and perpendicular to the stroke axis. Each flexure spring includes four independently flexible spring fingers extending radially outward in the same plane at ninety degrees from each other. The four spring fingers of each flexure are arranged in two pairs of opposite spring fingers each pair extending radially outward at a right angle. Two of the four pairs of spring fingers have tapered tips of decreased thickness defining an angled contact surface for contacting a fixed structure in the housing. As the armature is translated along the stroke axis toward the coil, the non-linear spring provides piece-wise forces acting on the armature by flexure of the four pairs of spring fingers at different portions of the armature stroke by contact of the spring fingers of the first flexure spring with the fixed structure and contact of the spring fingers of the second flexure spring with the corresponding spring fingers of the first flexure spring. This arrangement provides four distinct spring forces applied against the armature at various portions of the stroke.
An elongated axial rod can be attached to the armature and a compression spring can be disposed about the rod to bias the armature away from the coil. The compression spring provides a fifth distinct spring rate acting against the armature.
The metering valve assembly includes a generally cylindrical guide fixed to the housing along the stroke axis. The guide has inlet and outlet openings in fluid communication with respective the inlet and outlet ports of the housing. A cylindrical valve member can slide along the diameter of the guide to alternatively block the inlet and outlet openings in the guide and thereby control flow through the device. A valve carrier is disposed about the valve and has an annular flange surface engaging an outer circumferential shoulder of the valve. A compression spring, retained by an annular spring retainer fixed to the guide, is disposed about a portion of the valve to engage the shoulder and bias the valve toward, and the valve carrier in contact with, the armature.
In another form, the device includes an electronic control unit for controlling the operation of the coil. The device can also include a position transducer electrically coupled to the control unit and having a sensing coil and a metallic transducer element fixed to the elongated rod and disposed axially within the sensing coil of the position transducer. The position transducer provides a feedback signal to the control unit corresponding to the position of the transducer element. The control unit can adjust the input signal supplied to the coil in response to the feedback signal. Preferably, the control unit includes a comparator which compares a commanded metering valve position to an actual metering valve position detected by the position transducer. The control unit adjusts the input signal until the difference between the commanded and actual position is an acceptable value near zero.
Thus, the present invention provides a device for metering fluid in proportion to an input signal. This is accomplished using a simple and inexpensive non-linear spring assembly having two flexure springs with pairs of spring fingers of different thickness acting independently in a piece-wise manner to oppose armature movement induced by the magnetic flux generated by the coil. The incrementally increasing spring forces produce opposing forces corresponding to the magnetic flux as the distance between the armature and the coil changes. Since the magnetic flux pulling force is not proportional to the input signal to the coil, the resulting force of the non-linear spring assembly provides a counteracting non-proportional force throughout the stroke of the valve, which linearizes the movement of the metering valve assembly so that it is proportional to the coil input signal (current). Moreover, the feedback signal of the position transducer can be used to adjust the input signal to rectify discrepancy between the actual position of the meter and the position corresponding to desired fuel flow. In this way, the device provides for accurate metering suitable for use in precision actuator positioning or fuel metering applications, such as jet and rocket engines.
These and still other advantages of the present invention will be apparent from the description of the preferred embodiments which follow.